The present invention relates to a method and apparatus for ensuring adequate and uniform cooling for a single heat-generating device or an array of discrete heat-generating devices.
Heat fluxes and temperature uniformity constraints for modern electronics and optics are progressively demanding. As this occurs, the cooling technology for these components shifts from low-cost legacy solutions like air cooling and liquid cooling that produce heat transfer coefficients of 100 s to 1000 s of W/m2K to advanced thermal management solutions, such as vapor-compression cooling that can supply heat transfer coefficients up to 100,000 W/m2K.
Vapor compression technology has a limitation, however, when applying it to an arrayed heat load with many discrete sources. Supplying cooling to discrete sources (or circuits in an evaporator) has been historically accomplished via distributors located immediately downstream of the expansion device. These devices impose a high fluid velocity (which translates to a considerable pressure loss) through discrete orifices within the device. Each discrete orifice is dedicated to each discrete heat load. The momentum-driven separation will give near uniform flow distribution between the conduits at the cost of pressure loss. As the heat rejection temperature approaches the cooling temperature, the high-side pressure and low-side pressure of the system also converge, therefore making less pressure drop available to be used for flow balancing and further limiting the flow control of the pressure-drop balancing approach. Additionally, as the number of discrete cold plates (heat loads) increases; the size, weight, and complexity of the plumbing necessary to balance the two-phase flow-mixture downstream of the expansion device (and upstream of the cold plates) increases dramatically. Without the refrigerant flow uniformity to the individual cold plates, the temperature uniformity on and between cold plates will degrade rapidly, causing component failure in some cases.
We have discovered, however, an improved way to control the temperature uniformity of discrete heat loads using a vapor compression cooling scheme by integrating individual compact thermostatic expansion devices (TXVs) and their control directly into each individual cold plate. Individual control of each cold plate simplifies the overall system because the cold plates can all be maintained at a uniform temperature, regardless of the heat load to each individual cold plate, simply by maintaining a uniform outlet pressure of the superheated refrigerant exiting the cold plates, and ensuring that the cold plates are all supplied with sub-cooled liquid refrigerant at each cold plate inlet. By way of the present invention, the action of the integral TXV (of the cold plate) via opening or closing an internal orifice controls the flow to each cold plate. Thus, the control of each cold plate is localized, greatly reducing the overall control complexity. In addition, the inlet and outlet headers to the arrays of cold plates are simplified because the inlet headers have only subcooled liquid and outlet headers have only superheated vapor. This approach avoids individual two-phase distributor lines that must be balanced and fed to each cold plate.
In addition to saving space, plumbing complexity, and control complexity for the overall system, integrating the TXV within the cold plate also provides much faster TXV response time. Integrating the TXV into the cold plate also allows the sensing element to be integrated into the TXV. Integrating the sensing element with the TXV eliminates the capillary tube used in conventional TXV systems and improves the time response of the TXV because the sensing element can be immersed in the exiting refrigerant stream instead of relying on conduction of the outlet tube. Finally, with the integrated TXV configured as a cartridge, sealing can be accomplished via O-rings or other mechanical seals, thereby enabling field replacement of individual TXVs without the need of brazing or soldering, as is required for some conventional TXVs. Because of its enhanced temperature control, ability to be field replaceable, and augmented response time, the present invention enables the use of advanced power electronics, and more specifically, high-power lasers that demand temperature uniformity on the order of ±2° C. and full load “turn-on” times of less than 400 milliseconds.
As is well known in the art, the conventional TXV approach, as disclosed, in U.S. Pat. No. 4,750,334 whose FIG. 2 is repeated here as FIG. 10, uses an external sensing bulb tilled with a temperature sensitive working fluid that is mechanically attached to the outlet of the evaporator. Temperature changes at the outlet of the evaporator are converted to changes in working fluid pressure within the external sensing bulb. The pressure within the external sensing bulb is communicated through a capillary tube 64 to a diaphragm which works against a resistive force to open the valve. This is also discussed in U.S. Pat. No. RE23,706 where an “equalizer passage” (col 3, lines 36-45) communicates the pressure at the inlet of the evaporator to the underside of the follower 22, and of the opposite side of the pressure created by the sensing bulb 10.
U.S. Pat. No. 4,712,384 sought to integrate the TXV into the evaporator and thereby improve the conventional approach by integrating the sensing bulb and diaphragm into the outlet stream of the evaporator. However, that approach used an external mechanical “push rod” 82 to actuate the valve portion of the TXV, which was located in the inlet stream of the evaporator as seen in FIG. 4 and shown in the present application as FIG. 11. We have discovered a far better approach, which is more compact, simpler, and does not require the use of an external push rod to activate the opening of the orifice of the TXV. A further concern with the external push rod approach is that, as temperatures change, the TXV valve opening is altered by the thermal expansion of this relatively long push rod. Since an air-to-refrigerant evaporator can tolerate both an imprecise superheat control and a large variation in surface temperature, this may not be a serious thermal performance drawback, but it is certainly expensive to manufacture and maintain. For advanced cold plate thermal control, as envisioned by the inventors named in this application, and as stated previously, a precise thermal control, namely temperature uniformity on the order of ±2° C. and full load “turn-on” times of less than 400 ms, are required. The conventional approach described in U.S. Pat. No. 4,712,384 does not have this capability.
U.S. Pat. No. 5,297,728 discloses another attempt to eliminate the sensing bulb of the TXV and whose FIG. 1 is reproduced herein as FIG. 12. This prior art proposed an external TXV, which differs from a TXV integrated into the cold plate of the evaporator as described in this application. Rather than utilizing a sensing bulb mounted to the exterior of the outlet tubing, said patent proposed passing the outlet refrigerant through an added passage in the TXV and incorporating the temperature-activated force to open and close the valve within this added passage. While, as noted above, the prior art had also proposed a “push rod” to transmit this force. U.S. Pat. No. 5,297,728 used a valve body member 22 with an absorbent gas mixture to provide a “heat ballast” (col 4, lines 56-64). The present invention does not, however, require any complex mechanism to dampen the TXV response and avoid overshoot, or undershoot since the thermal inertia of the exiting superheated refrigerant and the surrounding metal of the cold-plate are utilized to provide the thermal ballast.
An object of the present invention is to provide a cartridge TXV that fundamentally differs in the method of temperature and refrigerant flow control that has been used in the past. First, the sensing element is integral to the TXV but is located external to and at the end of the TXV instead of within the TXV. Second, the integrated TXV is inserted into the evaporator outlet flow path. These two key differences eliminate two fluid connections and temperature variations experienced within the sensing bulb of U.S. Pat. No. 5,297,728. Third, having the sensing element immersed in the outlet stream of the evaporator and integral to the TXV enhances control because it eliminates the conduction resistance seen in conventional TXV arrangements.
An additional object of the present invention is to provide a method for improving and localizing the control of evaporative cold plates, while reducing the complexity of the remainder of the vapor compression system. The present invention goes beyond previous efforts to integrate TXVs into cold plates for vapor compression systems. U.S. Pat. No. 8,312,736 discloses an integrated TXV in a reverse-flow double pipe configuration. In this known configuration partially expanded refrigerant maintained above the ambient dew point flows through the outer pipe and then through an expansion device. Once through the expansion device, the cold refrigerant accepts heat from an electronics load and flows out through the inner pipe. This design seeks to eliminate condensation from sub dew-point cooling of electronics. The major drawback to this approach, however, is that the expansion is performed via fixed orifice (capillary tube) incapable of controlling the cooling temperature or the superheat into the compressor. Therefore, precise temperature control of sensitive electronics with varying ambient conditions and thermal loads cannot be performed with this device. Taking a different approach, the present invention integrates a cartridge TXV into a cold plate to provide active temperature control of sensitive electronics and eliminate the drawbacks of previous attempts.